This invention relates to a disk drive unit which automatically changes a plurality of disks to read-access (or access to reproduce) or read/write-access a disk. More particularly, the present invention relates to a disk drive unit equipped with a plurality of disk driving mechanisms.
In Japanese Non-examined Patent Publication No. 5-342740 (1993) as illustrated in FIG. 13, there is disclosed a conventional automatic disk changing equipment (a first conventional equipment) consisting of a rack 52 which revolves on its central shaft 51 with a plurality of disks 50 arranged on it in a ring shape with the central shaft 51 as the center, a plurality of disk transfer mechanisms 53a through 53c each of which transfers and reverses a disk 50, and a plurality of disk driving mechanisms 54a through 54c each of which drives (read-accesses or read/write-accesses) a disk 50, wherein said disk transfer mechanisms 53a through 53c are respectively on the disk driving mechanisms 54a through 54c. 
Further, in Japanese Non-examined Patent Publication No. 2-14460 (1990) as illustrated in FIG. 14, there is disclosed a conventional automatic disk changing equipment (a second conventional equipment) consisting of a stationary rack 56 which piles up a plurality of disks 55, a disk transfer mechanism 57 which transfers and reverses a disk 55, and a plurality of disk driving mechanisms 58a and 58b each of which drives a disk 55, wherein said disk driving mechanisms 58a and 58b share the disk transfer mechanism 57.
Said automatic disk changing equipment drives a plurality of disks 50 or 55 simultaneously by said disk driving mechanisms 54a through 54c or 58a and 58b. 
In the first conventional automatic disk changing equipment, however, it may rarely happen that two or more disks 50 are transferred at a time. Therefore, it is not recommendable to provide dedicated disk transfer mechanisms 53a through 53c respectively on the disk driving mechanisms 54a through 54c judging from a point of cost and space reduction. Further they occupy some installation spaces, which prevents installation of additional disk driving mechanisms.
In the second conventional automatic disk changing equipment having only one disk transfer mechanism 57 to transfer disks 55 between the stationary rack 56 and the plurality of disk driving mechanisms 58a and 58b, disks must be transferred a long way and consequently, it takes more time to change disks.
Accordingly, it is an object of the present invention to provide a compact disk drive unit which can change disks over a plurality of disk driving mechanisms without the aforesaid problems of the conventional automatic disk changing equipment.
The disk driving unit in accordance with the present invention comprises a rack which revolves on its central shaft with a plurality of disks arranged on it radially with the central shaft as the center, a plurality of disk driving mechanisms each of which drives (read-accesses or read/write-accesses) a disk, an access mechanism which is located between said rack and said disk driving mechanisms to transfer said disks between thereof, and a control unit. Said access mechanism is pivotally supported by a second central shaft which is approximately in parallel with said first central shaft and equipped with a disk transfer means which can selectively face to said rack and to any of said disk driving mechanisms. Said disk driving mechanisms are disposed radially around said second central shaft to face to the revolving access mechanism at a preset distance.
A feature of the present invention is that said disk transfer means takes in a disk from one side of the means facing to said rack and pushes it out from the identical side into one of said disk drive mechanisms.
Another feature of the present invention is that said disk transfer means takes in a disk from one side of the means facing to said rack and pushes it out from the other side into one of said disk drive mechanisms.
A further feature of the present invention is that said disk driving mechanisms are located around said second central shaft to face said access mechanism and that said access mechanism is built up so that it may take in a disk from one of said disk driving mechanisms and push it out into the opposite disk driving mechanism.
According to the present invention, an access mechanism for transferring a disk between said rack and said disk driving mechanisms is provided so that the access mechanism can revolve on a central shaft which is approximately in parallel with the central shaft of the rack. Further, a disk transfer means is provided to transfer a disk to and from a selected one of said rack and said disk driving mechanisms which is opposite to the disk transfer means. Said disk driving mechanisms are radially disposed around the central shaft of said access mechanism so that they may concentrate to said second central shaft of said access mechanism with a preset distance from thereof.
Said access mechanism takes in a disk from said rack, revolves to select and face to one of said disk driving mechanisms around it, and pushes the disk into the selected disk driving mechanism.
These features of the disk drive unit in accordance with the present invention make the unit smaller and disk changing faster.